I queried that exact point with ATC last week, as well as others.
Reply was safety concern due Tarmac works adjacent to bays 19 -21.
I'm still scratching my head as to where those works preclude the use of RWY 03 & 06 for departures
Have a think about how aircraft get from QF/Skippers/Network/Cobham to the threshold of 06. It looks like it
could be done but, would it be worth the hassle/increased complexity?